The word used in the example below is ‘propinquital’ – but that is not listed in any dictionary I can find, so the definition below is for ‘propinquity’. This adjectival form seems to be unique to Peake. There are a bare handful of words ending with -quity, but many of those have adjective forms ending with -quitous rather than -quital: ‘ubiquity’ and ‘ubiquitous’, for instance. Which suggests that Peake should have used ‘propinquitous’ instead.

pro·pin·qui·ty
/proʊˈpɪŋkwɪti/

–noun
1. nearness in place; proximity.
2. nearness of relation; kinship.
3. affinity of nature; similarity.
4. nearness in time.

Origin:
1350–1400; Middle English propinquite < Latin propinquitās nearness, equivalent to propinqu ( us ) near ( prop ( e ) near ( see pro- 1 ) + –inquus adj. suffix) + –itās -ity

Source: Dictionary.com.

As objects of beauty, these works held little interest to him and yet in spite of himself he had become attached in a propinquital way to a few of the carvings.

Source: Titus Groan by Mervyn Peake.